This invention relates to a mobile communications network, in particular but not exclusively to a cellular radio communications network, such as a GSM (Global System for Mobile Communications) digital cellular radio network.
A known conventional GSM network, referred to as a public land mobile network (PLMN), is schematically illustrated in FIG. 1. A mobile switching centre (MSC) 2 is connected via communication links to a number of base station controller (BSCs) 4. The BSCs 4 are dispersed geographically across areas served by the mobile switching centre 2. Each BSC controls one or more base transceiver stations (BTSs) 6 located remote from, and connected by further communication links to, the BSC. Each BTS 6 transmits radio signals to, and receives radio signals from, mobile stations 8 which are in an area served by that BTS. That area is referred to as a xe2x80x9ccellxe2x80x9d. A GSM network is provided with a large number of such cells, which are ideally contiguous to provide continuous coverage over the whole network territory.
A mobile switching centre 2 is also connected via communications links to other mobile switching centres in the remainder of the mobile communications network 10, and to a public service telephone network (PSTN), which is not illustrated. The mobile switching centre 2 is provided with a home location register (HLR) 12 which is a database storing subscriber authentication data including the international mobile subscriber identity (IMSI) which is unique to each mobile station 8. The IMSI is also stored in the mobile station in a subscriber identity module (SIM) along with other subscriber-specific information.
The IMSI consists of a string of digits, including a mobile country code, indicating the home country of the mobile, a home public land mobile network (PLMN) code, indicating the home mobile network of the mobile station, an HLR code indicating the identity of the HLR in which the subscriber information is stored, and a subscriber identity number identifying the subscriber in the HLR.
The mobile switching centre is also provided with a visitor location register (VLR) 14 which is a database temporarily storing subscriber authentication data including the temporary mobile subscriber identity (TMSI) which uniquely identifies a mobile station in the VLR, for mobile stations which are active in the area served by the mobile switching centre 2. Before subscriber authentication data is entered in a VLR, the subscriber is authenticated by accessing the subscriber""s HLR.
Because more than one GSM public land mobile network may operate in a particular territory, each such network is assigned a unique home PLMN code.
A mobile station 8 may move between cells served by the same or different mobile switching centres belonging to the same network without interruption in service. All mobile stations 8 in this known GSM mobile communications network, provided they are current subscribers, generally have access to the entire network.
However, in the event of congestion in a particular cell, a network operator may employ a cell access control mechanism provided for by the GSM standard (see GSM technical specification 3.22). On subscription, one or more access control classes are allocated to the subscriber and stored in the SIM. Access to a cell by certain subscribers may be prevented by omitting the corresponding access control classes from access control information broadcast by the cell BTS. This does not however prevent the excluded mobile stations attempting to register with the cell. Excluded mobile stations tend to lock onto an access-controlled cell without receiving service for significant periods of time. This access control mechanism is therefore rarely used by the GSM network operators.
Cordless telephone systems for use in both large and small business concerns and residential applications are known. One such system is the DECT standard, which provides for communication between mobile and fixed parts of a radio system via a radio interface. A DECT cordless private branch exchange (PABX) can be connected to a GSM network to receive calls from the GSM network. Such an arrangement is described in international patent publication number WO 95/01074. In such an arrangement the cordless telephone is not compatible for direct access to the main GSM network, since it uses DECT signalling protocols which are different to GSM signalling protocols. The private branch exchange performs protocol interfacing to allow a call to be routed from the GSM public land mobile network to the cordless telephone. Thus, the cordless telephone can only be used close to the PABX.
U.S. Pat. No. 5,448,619 describes the provision of a private cellular mobile communications system within an area served by a public cellular system. The strength of the control channel for the private cellular system is increased to attempt to ensure that mobile stations within the coverage area of the private system attempt first to register on the. private system. Public system mobile units are turned away when attempting to register. The private cellular system includes a receiver for receiving broadcast control signals from the public cellular system. The private system is otherwise provided with independent means for radio interfacing, subscriber authentication, call management, etc.
British patent publication number 2285556 describes a personal base station which is compatible with the GSM system. The personal base station is connected directly to a single telephone line in a PSTN, or ISDN, and a limited number of GSM functions are performed by the personal base station. This includes keeping a list of allowed mobile station numbers which have access to communications via the personal base station and register with it. The personal base station is provided with a personal cell identification which is entered in a PLMN list for the mobile station, so that an authorised mobile station will lock onto the broadcast control channel of the personal base station. This arrangement is intended only to support a single mobile communications call connection at any one time. Thus, the mobile station acts as a cordless telephone when close to the personal base station, or a mobile telephone when away.
A proposal for xe2x80x9cparallel network architecturexe2x80x9d has been made for use in the GSM system, as described in xe2x80x9cMobile Radio Technologyxe2x80x9d, Gordon White, published by Butterworth/Heinemann Limited in 1994. According to this proposal, physical components of a GSM system can be shared by different network operators. The different operators, which use different network identity (PLMN) codes, may share a base station, and the shared base station will transceive at all the different frequency bands for which the operators are respectively licensed. Mobile stations therefore communicate exclusively with the frequencies which their own operator utilises. It will be appreciated that this arrangement relates to two different networks, which have one or more shared physical components in common.
According to the present invention a mobile communications network is provided which comprises a main network portion assigned a first network identity code for communicating with a mobile station, and an additional network portion assigned a second network identity code, different to said first network identity code, for communicating with said mobile station said network identity codes being of the type used by said mobile station for distinguishing said mobile communications network from different mobile communications networks.
Herein, the term xe2x80x9cnetwork portionxe2x80x9d is intended to mean a part of the mobile communications network, whereby communications can be established with a mobile station. Thus, for example, in a GSM-type system, a network portion will generally include one or more BTSs.
An advantage of the invention is that the benefits of a mobile communications network can be provided for the additional network portion as well as the main network portion, whilst the two different portions can conveniently be distinguished by the use of different network identity codes.
Preferably, the additional network portion rejects from service mobile stations using the first network identity code. This allows the additional network portion to be configured as a private communications facility for use by mobile stations using the second network identity code only, thereby relieving congestion in use of the private, communications facility.
Conversely, the main network portion preferably accepts for service either mobile stations using the first network identity code or mobile stations using the second network identity code. This allows the main network portion to be made available for use by mobile stations using the second network identity code. Thereby, mobile stations using the first network identity code can obtain service throughout the network coverage. In addition, insufficient capacity problems in the additional network portion can be alleviated by servicing overflows via the main network portion.
The main network portion and the additional network portion preferably operate using frequencies within the same operator allocated frequency group, which allows the network operator to utilise the same frequency band or bands for which it is licensed as a main network operator, in the additional network portion.
In a cellular system, the main network portion is preferably adapted for handing over a mobile station from the additional portion to the main network portion and/or from said main network portion to said additional portion, when a call is in progress. Calls can then be continued even when a mobile station moves outside the range of one of the network portions.